2 Answers
2

According to the various answers found here and on Wikipedia, soak testing seems to be a test of normal sustained use for a long period of time. This is done to to ensure bugs or memory leaks do not appear after what is considered to be a relatively "normal" usage period.

Stress testing is also a form of reliability test that tests beyond normal usage of the application for a shorter time to see if it breaks or not. In that category you can typically find testing how the application behaves when a lot of concurrent users are connected and system resources start to lack (memory, processing time, bandwidth, etc.)